OH NO! There has been a devastating apocalypse that has ruined Earth and resulted in monsters being created and attacking us! What will we do to survive?
I know what we can do! Let's move into a cave! It will protect us from the deadly monsters that now live on our planet. But, there is not enough room down here and, it gets to be really dark. We will have to dig further.
Look! I have found a ball of light. It lights up the space, so it is helpful, but it is also really pretty. I have an idea, we can use the light as a type of currency. It will be commodity money that we will use as a source of light and a method of payment while we are down here.